Heretofore, the mechanical drive from a helicopter turbine engine to the helicopter transmission has involved a floating shaft. That is, the shaft is coupled through a spline on the output of the turbine to a crown gear at one end and through a crown gear to splines on the other end. The drive shaft is restrained normally in a mean position but is permitted to move axially, opposing normally balanced spring forces.
In operation of a crown gear-spline unit, lubricant is required to be maintained to avoid undue wear. In prior shaft designs the sealing arrangement to prevent leakage of the lubricant was positioned axially away from the pivotal plane of the couplings, causing relative lateral displacement of the outer and inner couplings when misaligned. Relatively large motion makes it impossible to use lip-type seals and other sealing arrangements have been found to be difficult. Rubber boots have been used on some drive shafts, but boot life generally is found to be low and failures result. This is because shaft design requires the boots to be located well away from the pivotal axis of the coupling so that they must flex through large amounts of motion with each revolution of the shaft. Sliding rings have also been used, but these, too, necessarily have been located away from the pivotal plane and have suffered large amounts of motion resulting in rapid wear rates.